Chromophore rotation in 124-k Dalton Avena sativa phytochrome as measured by light changes in linear dichroism

• From light‐induced changes in linear dichroism, we have calculated the rotation of the long‐wavelength‐absorbing transition moment that occurs during phototransformation of 124‐kilodalton Avena sativa phytochrome. Phytochrome was purified to homogeneity and immobilized onto Sepharose beads covalently coated with antibodies against A. sativa phytochrome. Changes in linear dichroism were induced by plane‐polarized red or far‐red light and measured by the absorbance differences at 660 and 730 nm using a dual‐wavelength spectrophotometer equipped with polarizing filters in the measuring beams. From such measurements, we calculate a rotation angle of 31o (or 149o) during photoconversion of Pr to Pfr and 30o (or 150o) during photoconversion of Pfr to Pr. These values are similar to the value of 32o (or 148o) reported earlier for the rotation of the transition moment of “large”A. sativa phytochrome (? 120 kilodalton) isolated under conditions that did not preclude post homogenization proteolysis of the 124 kilodalton molecule.

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